U.S. Pat. No. 5,156,738 to Maxson, which is incorporated herein by reference in its entirety, describes an underdrain system having a curved screen situated above a flat base plate. The underdrain system positions below a media bed so that liquid, usually water, exiting overflow troughs above the media bed can pass through the media.
During normal operation, water that has passed through the media bed falls into the underdrain system and into the screen, which has thin slots sized to retain particles that make up the media bed. Yet, water passing through the screen can be subsequently discharged from the underdrain system. As opposed to normal operation, a backwash mode can be used clean the media. In the backwash mode, water and air are directed upwardly through the screen and media bed in a uniform manner so that the material previously filtered out and captured by the media bed can be discharged through an overflow trough.
The underdrain system is formed of stainless steel and has its screen portion situated between two up-turned, side edge portions of its elongated flat base plate. Its screen portion is formed by helically winding and welding a metal wire to a channel base support as described in Geske U.S. Pat. No. 4,096,911, for example.
Screens and base plates that form lateral assemblies for underdrain systems can be quite large, often measuring as long as 30 feet, but they are more typically about 20 feet long. In the prior art, such lateral assemblies as described above have been formed of metal. Accordingly, these assemblies are difficult to transport and assemble due to their weight. Furthermore, where these metal assemblies are to be used with seawater or brackish water, they are subject to corrosion even when fabricated of stainless steel. In addition, because stainless steel is quite expensive, the base plates of these metal assemblies are usually relatively thin, allowing them to bow during use unless a substantial number of fixtures are provided to keep them in place and flat against the basin floor.
To overcome some of the challenges associated with such a metal lateral assembly, another underdrain screen is disclosed in U.S. Pat. No. 5,618,426, which is incorporated herein by reference in its entirety. As shown in FIGS. 1A-1B, the lateral screen 10 has a curved screen element 20 and a rigid extruded base plate 50. The lateral screens 10 can be similar to the lateral screens of Johnson Screen's Triton® Underdrain System.
This screen 10 can be used in water treatment applications. When installed at the bottom of a water treatment basin, the lateral screen 10 collects and distributes water while directly retaining filtering media. The base plate 50 has a central opening 54 for a fitting seal 55 composed of nitrile or neoprene rubber, and the lateral screen 10 can be closed at its ends by molded plastic end caps 80. When installed in a basin, the fitting seal 55 fits onto the end of a drop pipe (not shown) passing through the floor of the basin.
The screen element 20 slides into channels 52 along the longitudinal edges of the base plate 50, which is composed of extruded polyvinylchloride (PVC). The screen element 20 is composed of PVC wire 40 (e.g., Vee-Wire®) wound around and welded to “U” shaped channel rods 30 at each intersection to the channel ribs 34. (VEE-WIRE is a registered U.S. trademark of Weatherford/Lamb, Inc.) The screen element 20 can also be made of other materials, such as stainless steel or as required by the process in which the screen will be used.
On the screen element 20, the wires 40 and ribs 30 define small openings for the retention of media and collection of filtered liquid. During fluid flow, the small openings cooperate with each other to form a flow distribution system for uniformly controlling the flow of water and air passing into and out of the screen element 20.
In some implementation, it is desirable for the underdrain screen 10 to have an extended length. However, such a longer screen 10 needs additional leveling and hold down capability due to higher variations in the flatness of the basin floor in the larger basins the longer screens 10 will be installed. Typically, this will be for larger water treatment or desalination plants.
In smaller installations, the variation on the floor height of the basin is typically specified to be within ¼″. This has allowed the underdrain screens 10 to be successfully installed and used without the need for sophisticated leveling. For larger basins, however, it can be difficult to maintain the ¼″ tolerance in flatness. In some cases, shims or other spacer systems must be placed under the underdrain screens 10 by installers.
Stainless steel underdrain screens can have a leveling screw welded to the side of the screens to allow them to be leveled for larger basins. The tops of these leveling screws are held down by a channel, which both holds the underdrain screens in place and prevents the leveling screws from moving during operation. This form of leveling and hold down is not available for screens made of plastic, such as PVC.
Instead, as shown in FIG. 1C, the common way of leveling and retaining underdrain lateral screens 10 of plastic in a basin B uses shims 70 and upper hold-down bars 60. Operators fit the shims 70 underneath the screens 10 where needed along their length to level them. To hold down the screens 10, operators install the angle hold-down bars 60 over the top of the lateral screens 10. These bars 60 affix by anchor bolts 65 in the basin floor and run perpendicular to the length of the lateral screens 10.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.